Implement rate-limiting

http://www.vinaysahni.com/best-practices-for-a-pragmatic-restful-api#rate-limiting
not totally sure about this, but maybe we should inject the restful handler into the class. So get/set account would be ``$this->getRestfulHandler()->getAuthenticationManager...``

Advantage: is that we could use  ``$this->getRestfulHandler()->addHttpHeader()`` to set the header instead of directly calling ``drupal_add_http_header()``

Disadvantage: More coupled with the Restful handler, although I can't think of a use case where the rate limit would be by itself.